Yeah, drinking a gallon of anything that fast is no fun, but the laxative they gave me was terrible. The worst part was that I tried drinking it cold, which made it sit in my stomach and build up, so I ran out of space to drink more. I heated it up, which got things moving again, but made it taste even worse.
I will say though, after having the colonoscopy, my general bowel symptoms(never ending gas, mostly) improved dramatically. I'm thinking about doing a chemical cleanse every year just to keep things working fine.